In general, there are many steam turbine plants having so-called three-casing axial-flow type structures in which a high pressure steam turbine, an intermediate pressure steam turbine, and a low pressure steam turbine are directly connected to a shaft of a turbine rotor so that a power generator generates as much load (output) as possible.
The steam turbine plant having the three-casing axial-flow type structure is configured to house a plurality of turbine stages, in which a turbine nozzle and a turbine blade are combined in a turbine casing of each of the high pressure steam turbine, the intermediate pressure steam turbine, and the low pressure steam turbine, along an axial direction of the turbine rotor. Further, the steam turbine plant performs expansion work at the turbine stage of the high pressure steam turbine with steam that is supplied from a boiler, and after the steam completes the expansion work, the steam is again heated by a reheater of the boiler. Then, the reheated steam is supplied to the low pressure turbine via the intermediate pressure turbine, and the steam drives the power generator by performing the expansion work at the turbine stage of each of the steam turbines. Thereafter, the steam is condensed into water by a condenser after completing the expansion work, and the condensed water is recycled by a supply water heater to serve as supply water. Consequently, the supply water is again fed back to the boiler.
In addition, the steam turbine plant is provided with a numbers of valve devices having different diameters in accordance with their function and use.
In these valve devices, in particular, for example, both of a main steam shut-off valve and a steam control valve (steam flow regulation valve) disposed between the boiler and the high pressure steam turbine are configured to be super-sized valve devices having a pressure-proof structure, because steam at very high pressures from 16.6 MPa to 24.1 MPa, and at very high temperatures from 538° C. to 566° C. is applied thereto.
At this moment, for example, the main steam shut-off valve is a valve of an ON-OFF type that immediately supplies the steam toward the high pressure steam turbine when commencing operation, and that immediately shuts off when the load is shut off. In addition, for example, the steam control valve (steam flow regulation valve) is a valve of control valve type that controls the flow rate by opening a valve body at an arbitrary valve-opening amount in response to a demand of the load and that immediately shuts off the valve body when the load is shut off.
Heretofore, the above-described main steam shut-off valve and the steam control valve (steam flow regulation valve) are disposed at separate positions before-an inlet of the high pressure steam turbine. However, because both the size of the main steam shut-off valve and the size of the steam control valve (steam flow regulation valve) are extremely large, a large space for installing the main steam shut-off valve and the steam control valve (steam flow regulation valve) has been required.
However, recently, a so-called steam valve of combined type has been realized. That is, a reduced-size steam valve, requiring a small installation space by combining the main steam shut-off valve and the steam control valve (steam flow regulation valve) and housing the main steam shut-off valve and the steam control valve (steam flow regulation valve) in one valve casing, has been realized. An example of the configuration of the combined type steam valve is shown in FIGS. 30 through 32.
FIG. 30 is a conceptual diagram showing a known steam valve in which the main steam shut-off valve and the steam control valve (steam flow regulation valve) are housed in the valve casing, FIG. 31 is a diagram showing a main steam flow in a known steam valve in which the main steam shut-off valve and the steam control valve (steam flow regulation valve) are housed in the valve casing, and FIG. 32 is a cross-sectional view of FIG. 30 looking from arrows XXXII-XXXII.
For example, in a steam valve 1 composed of the main steam shut-off valve and the steam control valve (steam flow regulation valve) combined with each other, a first valve device 2 that corresponds to the main steam shut-off valve is disposed at an upstream side of the main steam flow, and a second valve device 3 that corresponds to the steam control valve (steam flow regulation valve) is disposed at a downstream side of the main steam flow, and in addition, the first valve device 2 and the second valve device 3 are housed in a valve casing 4.
The first valve device 2 is provided with a first main steam inlet 5 and a first main steam outlet 6 in the valve casing 4, which is connected to a second main steam inlet 7 of the second valve device 3, and the first valve device 2 houses a strainer 8 that removes impurities such as oxidized scale and the like.
Further, the first valve device 2 is provided with a first valve body 10 that detachably contacts a first valve seat 9 provided at a side of the first main steam outlet 6, and a first driving device 12 that drives the first valve body 10 to freely travel forward and backward via a first valve rod 11.
On the other hand, the second valve device 3 is provided with a second valve body 16 that detachably contacts a second valve seat 14 provided at a side of the second main steam outlet 13 and slides in a sleeve 15, and a second driving device 17 that drives the second valve body 16 to travel forward and backward via a second valve rod 17.
At the steam valve 1 having such a configuration as mentioned above, when the main steam supplied from the first main steam inlet 5 to the valve casing 4 passes, from outside to inside, through the strainer 8, which has a plurality of tiny holes, as shown in FIGS. 30 and 32, the impurities such as oxidized scale and the like are removed, and thereafter, the main steam flows along the first valve rod 11 and is further supplied to the high pressure steam turbine via the second main steam outlet 13 of the second valve device 3.
The steam valve 1 having such a configuration can be of course reduced in size because the first valve device 2 and the second valve device 3 are housed in one valve casing 4. In addition, the first valve device 2 of the steam valve 1 has a function to instantaneously shut off the main steam in an emergency and the second valve device 3 has a function to control the flow rate. As a result, the steam valve 1 is able to immediately respond to any one of starting operation, rated load operation, partly rated load operation, and emergency shutting-off operation of the steam turbine plant.
Further, the steam valve in which the steam shut-off valve and the steam control valve (steam flow regulation valve) are housed in one valve casing while being combined with each other is disclosed in, for example, Japanese Patent Laid-open (KOKAI) Publication No. 2002-97903.
The steam valve 1 that is configured to have, for example, the steam shut-off valve and, for example, the steam control valve (steam flow regulation valve) combined with each other has plenty of advantages, as described above. On the other hand, the steam valve 1 provides a number of problems, and one of the problems is a reduction of a pressure loss.
In the known steam valve 1, shown in FIGS. 30 through 32, a secondary flow occurs due to a drift, and other main steam flows into an area of the secondary flow where deficiency of flowing speed exists. Then, a swirling flow occurs when flowing out from the second valve device 3. This is because the steam valve 1 has a main steam path of the first valve device 2, the main steam path of the second valve device 3, and more than one acute-angle bend. Thus, the pressure loss has been further increased due to the swirling flow.
The pressure loss based on the swirling flow has also been confirmed according to experimented data, such as a fluid analysis values, tests, and the like.
Further, it is found that, in this kind of steam valve of the combined type, the main steam supplied from the first main steam inlet 5 is separated into two flows at an inlet side of the strainer 8, and passes around a surface thereof. Then, two steam jets join together at an outlet side of the strainer 8 and collide with each other, resulting in the occurrence of a large mixing loss.
Furthermore, it is also found that because the main steam passing through the strainer 8 passes through multiple holes formed in the strainer 8 in order of precedence, a component of the speed in an axial direction in an internal space thereof is induced and thereby, the strong secondary flow occurs.
Thus, with the known steam valve of the combined type, it is difficult to suppress the pressure loss caused by factors such as the secondary flow, the mixing loss, or the like.
In general, it is said that when the pressure loss of the steam valve is decreased by 1%, a heat rate of the steam turbine plant improves equal to or more than 0.1% (Literature: Turbomachinery Vol. 30 No. 7), and in light of an improvement of heat efficiency of the plant, a pressure loss reduction in the steam valve has become an important matter that cannot be disregarded.
The present invention is made in light of the above described background art, and an object of the present invention is to provide a steam valve in which a further reduction of pressure loss is realized by effectively controlling the main steam flow at the strainer housed in the valve casing.